Automobile bodies and many industrial and consumer articles are conventionally spray painted in areas called spray booths, wherein water curtains are employed to cleanse the air of over-sprayed paint. The wash water is then treated to remove paint solids, and the treated water is recirculated.
A water curtain may be created by pumping water into a trough above the over spray area or by the use of spray nozzles. Fine droplets of over sprayed paint, emitted by a spray gun, contact and are captured by the water curtain. The amount of paint contacting a water curtain may change depending on a number of variables, including plant or process shutdowns, the size and shape of the object being painted, the type of spray equipment used, the spraying and purge technique used, and the water flow and the type of paint used.
In the past, solvent-based paints have commonly been employed in spray booths. However, in response to federal regulations limiting the amount of volatile hydrocarbons (i.e., the solvent diluent used in solvent-based paint) which can be emitted from a plant site, water-based paints are now being used in spray booth operations.
The term "water-based paints", as used herein, refers to all varieties of coatings which contain in excess of approximately 10% water in the coating formulation, including, but not limited to, water-reducible alkyl and epoxy ester compositions, water-borne thermoplastic latex compositions using acrylic polymer/copolymers, water-based latexes of polyurethane dispersions, and blends of such compositions. As used herein, the terms "water-based paints" and "water-borne paints" are synonymous.
When treating paint spray booth water that contains over-sprayed water-based paints, a primary objective is to capture and collect the finely dispersed paint solids. Uncaptured solids tend to accumulate in the system and settle in sludge recovery pits and booth weirs. Such solids encourage the growth of anaerobic bacteria colonies which may result in odor problems. This treatment problem is aggravated with paints that are water-based because they are more hydrophilic than solvent-based paints, i.e., they contain resins and dyes which are more compatible with water.
Other problems which severely interfere with spray booth operations occur in areas of high agitation where foaming occurs and in areas where foam accumulates. Foaming is caused by chemical additives, surfactants, solvents or combinations thereof. Finely dispersed paint solids which are not captured and removed tend to stabilize foam, which aggravates foaming problems. Foaming generally mandates that copious amounts of defoamers be used, which results in higher operating costs. Water-based paints generally tend to cause foaming to a greater extent than solvent-based paints.
A wide variety of chemicals have been proposed as treating agents for circulating wet spray booth waters containing over-sprayed paint, including compositions containing polymers and amphoteric metal salts which form insoluble hydroxides at pH's greater than about 7. The use of combinations of this type are described in the following U.S. Pat. Nos. 3,861,887 to Forney; 3,990,986 to Gabel et al.; 4,002,490 to Michalski et al.; 4,130,674 to Roberts et al.; and 4,440,647 to Puchalski. Further, U.S. Pat. No. 4,637,824 to Pominvllle discloses the use of silicates and polydiallyldialkylammonium halides with amphoteric metal salts, and U.S. Pat. No. 4,853,132 to Merrell et al. discloses the use of precipitates formed by the reaction of cationic polymers and salts of inorganic anions to detackify solvent-borne paints. Bentonite clays, aluminum salts and zinc salts have also been used with cationic polymers.
U.S. Pat. to 4,913,825 Mitchell discloses the use of dialkylamine-epihalohydrin polymers in combination with modified tannin and/or a melamine formaldehyde-type polymer in paint spray booth operations. This patent also mentions cationic starches at column 7, lines 10-15.
U.S. Pat. No. 4,656,059 to Mizuno et al. relates to the use of melamine-aldehyde acid colloid solutions for treating paint in wet spray booths; U.S. Pat. No. 4,629,572 to Leitz et al. relates to the use of urea or amino triazine-aldehyde condensation reaction products in combination with water-swellable clays to treat paint spray booth wastes; and copending applications U.S. Ser. No. 588,997 and 475,670 relate to melamine formaldehyde/alkalinity-based methods for treating oversprayed water-based paints.
The instant invention represents a novel approach relative to the inventions of the prior art in that cationic potato starch/hydrolyzed polyacrylamide-based systems are used to treat circulating paint spray booth waters containing over-sprayed water-borne paints.
Cationic starches are well known, particularly in papermaking applications. See, for example, U.S. Pat. Nos. 2,884,395, 3,674,725, 3,854,970 and 4,568,721.